A synthesis of a series of quanidinium substituted carboxylic acids and aminoguanidinium carboxylic acid analogs of hexamethonium and the examination of their binding, agonist/antagonist properties and effects on the kinetics and binding equilibria of alpha-neurotoxin to acetyl choline receptor in isolated membrane vesicles are proposed in order to test the hypothesis that the quanidinium carboxylate ion pair presumed to be formed between the side chains of an arginine (res 33) and an aspartic acid (res 31) residues of the sea snake alpha-neurotoxin is in part responsible for the tight, specific binding of the toxin to the post synaptic, nicotinic acetyl choline receptor. It is suggested that the proximity of these functional groups together with the desolvation of their microenvironment during the initial phases of the binding of this region of the toxin to the receptor facilitates the ion pairing interaction. By virtue of the electronic and sterochemical similarity of the ion pair to the acetyl choline molecule the former is suggested to become a major factor in the development of the strength and specificity of the toxin-receptor interaction (Tsenoglou, Petsko and Hudson, 1977).